Method of fabricating high-dielectric constant oxides on semiconductors using a GE buffer layer

ABSTRACT

This is a method for fabricating a structure useful in semiconductor circuitry. The method comprises: growing a germanium layer 28 directly or indirectly on a semiconductor substrate 20; and depositing a high-dielectric constant oxide 32 (e.g. a ferroelectric oxide) on the germanium layer. Preferably, the germanium layer is epitaxially grown on the semiconductor substrate. This is also a semiconductor structure, comprising: a semiconductor substrate; a germanium layer on the semiconductor substrate; and a high-dielectric constant oxide on the germanium layer. Preferably the germanium layer is single-crystal. Preferably the substrate is silicon and the germanium layer is less than about 1 nm thick or the substrate is gallium arsenide (in which case the thickness of the germanium layer is not as important). A second germanium layer 40 may be grown on top of the high-dielectric constant oxide and a conducting layer 42 (possibly epitaxial) grown on the second germanium layer. Preferably the high-dielectric constant oxide is a titanate, such as barium strontium titanate. When the high-dielectric constant oxide is a lead-containing titanate 34, a buffer layer of non-lead-containing titanate 32 is preferably utilized between the germanium layer and the lead-containing titanate.

BACKGROUND OF THE INVENTION

An arrangement of layers with an oxide between a conducting layers and asemiconductor is usable as a portion of many of the structures used insemiconductor circuitry, such as capacitors, MOS transistors, pixels forlight detecting arrays, and electrooptic applications.

The integration of non-SiO₂ based oxides directly or indirectly on Si isdifficult because of the strong reactivity of Si with oxygen. Thedeposition of non-SiO₂ oxides on Si has generally resulted in theformation of a SiO₂ or silicate layer at the Si ∥ oxide interface. Thislayer is generally amorphous and has a low dielectric constant. Theseproperties degrade the usefulness of non-SiO₂ based oxides with Si.High-dielectric constant (HDC) oxide (e.g. a ferroelectric oxide) canhave a large dielectric constant, a large spontaneous polarization, anda large electrooptic properties. Ferroelectrics with a large dielectricconstant can be used to form high density capacitors but can not bedeposited directly on Si because of the reaction of Si to form a lowdielectric constant layer. Such capacitor dielectrics have beendeposited on "inert" metals such as Pt, but even Pt or Pd must beseparated from the Si with one or more conductive buffer layers.

Putting the high dielectric material on a conductive layer (which iseither directly on the semiconductor or on an insulating layer which ison the semiconductor) has not solved the problem. Of the conductor orsemiconductor materials previously suggested for use next to highdielectric materials in semiconductor circuitry, none of these materialsprovides for the epitaxial growth of high dielectrical materials on aconductor or semiconductor. Further, the prior art materials generallyeither form a silicide which allows the diffusion of silicon into thehigh dielectric materials, or react with silicon or react with the highdielectric oxide to form low dielectric constant insulators.

The large spontaneous polarization of ferroelectrics when integrateddirectly on a semiconductor can also be used to form a non-volatile,non-destructive readout, field effect memory. This has been successfullydone with non-oxide ferroelectrics such as (Ba,Mg)F₂ but not sosuccessfully with oxide ferroelectrics because the formation of the lowdielectric constant SiO₂ layer acts to reduce the field within theoxide. The oxide can also either poison the Si device or create so manyinterface traps that the device will not operate properly.

Ferroelectrics also have interesting electrooptic applications whereepitaxial films are preferred in order to reduce loss due to scatteringfrom grain boundaries and to align the oxide in order to maximize itsanisotropic properties. The epitaxial growth on Si or GaAs substrateshas previously been accomplished by first growing a very stable oxide orfluoride on the Si or GaAs as a buffer layer prior to growing anothertype of oxide. The integration of oxides on GaAs is even harder than Sibecause the GaAs is unstable in O₂ at the normal growth temperatures 450C-700 C.

SUMMARY OF THE INVENTION

A Ge buffer layer directly or indirectly on Si oxidizes much lessreadily and can be used to prevent or minimize the formation of the lowdielectric constant layer. An epitaxial Ge layer on Si provides a goodbuffer layer which is compatible with Si and also many oxides. Unlikeother buffer layers, Ge is a semiconductor (it can also be doped toprovide a reasonably highly conductive layer) and is compatible with Siprocess technology. The epitaxial growth of Ge on top of theferroelectric or high-dielectric constant oxide is also much easier thanSi which makes it possible to form three dimensional epitaxialstructures. The Ge buffer layer can be epitaxially grown on the Sisubstrate allowing the high dielectric constant oxide to be epitaxiallygrown on the Ge and hence epitaxially aligned to the Si substrate. Theepitaxial Ge layer allows ferroelectrics to be directly grown on Siwafers to form non-volatile non-destructive read out memory cells. TheGe buffer layer will also increase the capacitance of large dielectricconstant oxide films compared to films grown directly on Si. A Ge bufferlayer on the Si or GaAs substrate allows many more oxides to beepitaxially grown on it because of the much smaller chemical reactivityof Ge with oxygen compared to Si or GaAs with oxygen.

Generally the prior art conductive materials suggested for interfacingwith high dielectric constant oxides in semiconductor circuitry eitherhave reacted with the high dielectric constant oxides or with thesemiconductor and/or have not provided a diffusion barrier between thehigh dielectric constant oxides and semiconductor material.

As, noted, the integration of oxides on GaAs is even harder than Sibecause the GaAs is unstable in O₂ at the normal growth temperatures ofhigh-dielectric constant oxide (450 C-700 C). A epitaxial Ge bufferlayer solves this problem and simplifies the integration offerroelectrics on GaAs for the same applications as listed above.

This is a method for fabricating a structure useful in semiconductorcircuitry. The method comprises: growing a germanium layer on anon-germanium semiconductor substrate; and depositing a high-dielectricconstant oxide on the germanium layer. Preferably, the germanium layeris epitaxially grown on the semiconductor substrate.

This is also a structure useful in semiconductor circuitry comprising: asemiconductor substrate; a germanium layer on the semiconductorsubstrate; and a high-dielectric constant oxide on the germanium layer.Preferably the germanium layer is single-crystal.

Preferably the substrate is silicon and the germanium layer is less thanabout 1 nm thick or the substrate is gallium arsenide (in which case thethickness of the germanium layer is not as important). A secondgermanium layer may be grown on top of the high-dielectric constantoxide and a conducting layer (any layer may be epitaxial if the layerbelow it is single crystal) grown on the second germanium layer.Preferably the high-dielectric constant oxide is a titanate, such asbarium strontium titanate. When the high-dielectric constant oxide is alead-containing titanate, a buffer layer of non-lead-containing titanateis preferably utilized between the germanium layer and thelead-containing titanate. Preferably the high-dielectric constant oxideis a ferroelectric oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the present invention will become apparent from adescription of the fabrication process and structure thereof, taken inconjunction with the accompanying drawings, in which:

FIG. 1 shows a cross-section of one embodiment of a multi-layerstructure using a polycrystalline Ge buffer layer;

FIG. 2 shows a cross-section of an alternate embodiment of a multi-layerstructure using a polycrystalline Ge buffer layer;

FIG. 3 shows a cross-section of an embodiment of a multi-layer structureusing an epitaxial Ge buffer layer; and

FIG. 4 shows a cross-section of an alternate embodiment of a multi-layerstructure using an epitaxial Ge buffer layer.

DETAILED DESCRIPTION OF THE INVENTION

As noted, the growth of HDC oxides on Si generally results in theoxidation of the Si and the formation of SiO₂ or a silicate layer.Further, this SiO₂ layer prevents the epitaxy of the deposited oxide andhas a low dielectric constant and the integration of ferroelectrics andother large dielectric constant materials directly on Si is degraded bythe formation of the low dielectric constant SiO₂ layer. Also as noted,putting the high dielectric material on a conductive layer (which iseither directly on the semiconductor or on an insulating layer which ison the semiconductor) has not solved the problem.

A Ge buffer layer between non-SiO₂ oxides and Si reduces the reactivityof the Si surface and in general enhances the epitaxy and at leastreduces the reaction layer between the deposited oxide and the Sisubstrate. The epitaxial growth of Ge on Si is compatible with currentSi process technology. The main difficulty with Ge on Si is the 4%lattice mismatch which results in misfit dislocation on Ge films thickerthan 1 nm. On silicon, the Ge layer is preferably very thin to avoid themisfit dislocations (however a thicker layer may be used for somedevices if that is not detrimental to the performance of the device inquestion). In still other embodiments, polycrystalline Ge may be formedover silicon dioxide or polycrystalline Si (thus using the Ge as achemical buffer layer between a deposited oxide and the Si substrate).

Depending on the application the choice of materials may be verydifferent. For large density capacitors, currently the best lineardielectric appears to be (Ba_(1-x),Sr_(x))TiO₃ (BST). BaTiO₃ (BT) orSrTiO₃ (ST) when deposited directly on Si forms a low dielectricconstant layer, because BT and ST are not thermodynamically stable nextto Si. Ge, however, has a much smaller free energy of oxidation and BTand ST are thermodynamically stable next to Ge. It is also possible todeposit BT and ST in a H₂ +O₂ gas mixture such that Ge is stable andalso BT or ST is stable while GeO₂ is not stable. Not all oxides arestable next to Ge. For example, all ferroelectrics containing Pb such asPb(Ti,Zr)O (PZT) are much less stable next to Ge (since PbO is notstable). A thin layer of SrTiO₃ or other stable ferroelectric can,however, be used as a buffer layer between the Pb containingferroelectric and the Ge coated Si substrate. The SrTiO₃ not only actsas a chemical barrier, but also nucleates the desired perovskitestructure (instead of the undesirable pyrochlore structure).

An epitaxial Ge buffer layer was used in experiments on a (100) Sisubstrate to deposit epitaxial BST. Without the Ge buffer layer, the BSTwas randomly oriented polycrystalline. With the Ge buffer layer, most ofthe BST has the following orientation relationship (110) BST ∥ (100) Si.This showed that the Ge buffer layer has prevented the formation of alow dielectric layer at the interface prior to epitaxy since that layerwould prevent epitaxy.

The deposition of a ferroelectric directly on a semiconductor has beenused by others to create a non-volatile nondestructive readout memory.This device is basically a MOS transistor where the SiO₂ has beenreplaced with a ferroelectric (metal-ferroelectric-semiconductor orMFS). One memory cell consists of a MFS transistor and a standard MOStransistor. This type of memory has many advantages including very fastread/write as well having nearly the same density as a standard DRAMcell. The remnant polarization in the ferroelectric can be used induce afield into the semiconductor and hence the device is non-volatile andnon-destructive. This device has been successfully made by others usinga (Ba,Mg)F₂ ferroelectric layer epitaxially grown by MBE on the Sisubstrate. Oxide perovskites such as PZT have also been studied fornon-volatile memories but these materials can not be deposited directlyon Si without reacting with the Si. A Ge buffer layer will allow manystable ferroelectrics, such as BaTiO₃, to be used in a RAM. A secondbuffer layer of SrTiO₃ or some other stable ferroelectric should alloweven most chemically reactive ferroelectric oxides to be used to try toform a RAM. The Ge buffer layer would also allow this type of memory tobe fabricated on GaAs and other III-V compounds in addition to Si. Italso might be possible to fabricate a thin-film MFS transistor bydepositing the Ge on top of the ferroelectric. The ferroelectric mightbe epitaxial on the GaAs or Si substrate or it might be polycrystalline.The compatibility of Ge with a stable ferroelectric buffer layer allowsthis structure to be manufactured.

In FIG. 1 there is shown one preferred embodiment (in all figures, anarrangement of layers is shown which is usable as a portion of manystructures used in semiconductor circuitry, such as capacitors, MOStransistors, pixels for light detecting arrays, and electroopticapplications). FIG. 1 shows a semiconductor substrate 10, on which ansilicon dioxide insulating layer 12 has been deposited, with a dopedpolycrystalline germanium layer 14 over the silicon dioxide 12 (thegermanium can be highly doped to provide a highly conductive layer, andthe germanium is polycrystalline, as it overlies an amorphous silicondioxide layer). A ferroelectric barium strontium titanate layer 16 isdeposited on the germanium layer, and a titanium-tungsten layer 18 isdeposited atop the barium strontium titanate 16. As noted, such anarrangement of layers is usable in many semiconductor structures and theferroelectric or high dielectric properties of the barium strontiumtitanate provides advantageous properties over most other insulatingmaterials.

FIG. 2 shows an alternate embodiment, again in which the germanium (andthus all layers above it) is polycrystalline. A silicon dioxide layer 12is on a silicon substrate 20 with a silicon nitride layer 22 atop thesilicon dioxide 12. A polycrystalline germanium layer 14, a bariumstrontium titanate layer 16 and a lead zirconium titanate layer 24 and atitanium nitride layer 26 are successively added atop the siliconnitride layer. In this case, the barium strontium titanate 16 generallyacts as a buffer layer to prevent reaction between the lead of the leadzirconium titanate 24 and the germanium 14.

FIG. 3 illustrates the use of epitaxially germanium. As the singlecrystal germanium 28 can be epitaxially grown on the gallium arsenidesubstrate 30, with a good lattice match, good crystal quality can beobtained both in the germanium 28, and the epitaxially barium strontiumtitanate layer 32 and on into the (Pb,Mg)NbO₃ layer 34. The topelectrode 18 can be titanium-tungsten (as in the preceding figures,annealing of the ferroelectric layer or layers can be accomplishedbefore deposition of the top electrode, and thus reactions between theferroelectric and a material such as titanium-tungsten can be used asreaction with the ferroelectric is minimized due to the low temperatureprocessing of the remaining steps). It should be noted that anepitaxially structure utilizing single crystal germanium allows thegermanium to be utilized as a part of a transistor, for example. Thus,single crystal germanium is useful, even if the ferroelectric materialis grown under conditions which provide a polycrystalline ferroelectric.

FIG. 4 shows an alternate structure using epitaxially germanium. Herethe silicon substrate 20 is covered by a germanium epitaxially layer 28,which is in turn covered by a barium strontium titanate epitaxiallylayer 32. A second germanium layer 40 is over the barium strontiumtitanate 32, and an aluminum top electrode 42 is over the secondgermanium layer 40. The use of a second germanium layer allows the usageof a wider variety of conductors for the top electrode and allows highertemperature processing during and after the deposition of the topelectrode, as the germanium generally prevents reaction between the topelectrode material and the ferroelectric material.

While a number of materials have been previously been suggested for usenext to high dielectric materials (such as barium strontium titanate orlead zirconium titanate), none of these materials provides for theepitaxial growth of high dielectrical materials on a conductor orsemiconductor. Further, the prior art materials generally either form asilicide (e.g. of palladium, platinum or titanium) which allows thediffusion of silicon into the high dielectric materials, or react withsilicon (e.g. tin dioxide) or react with the high dielectric oxide toform low dielectric constant insulators (e.g. titanium monoxide ortantalum pentoxide). Thus the prior art conductive materials suggestedfor interfacing with high dielectric constant oxides with semiconductorseither have reacted with the high dielectric constant oxides or with thesemiconductor and/or have not provided a diffusion barrier between thehigh dielectric constant oxides and semiconductor material. At theannealing temperatures necessary to produce good quality high dielectricconstant oxide material, such reactions generally form low dielectricconstant insulators, which being in series with the high dielectricconstant oxide material, dramatically lowers the effective dielectricconstant. Only germanium (doped or undoped) gives a conductor orsemiconductor which reacts neither with the semiconductor substrate northe high dielectric constant oxide at the required annealingtemperatures, and only germanium provides for epitaxial growth of aconductive or semiconductive material on a semiconductor substrate, in amatter compatible with growing and annealing of a high dielectricconstant oxide in a non-reactive manner, such that a metal oxide metalor metal oxide semiconductor structure can be fabricated without theeffective dielectric constant being significantly lowered by a lowdielectric constant material between the high dielectric constantmaterial and the underlying conductor or semiconductor.

Since various modifications of the semiconductor (e.g. silicon orgallium arsenide) structure, and the methods of fabrication thereof, areundoubtedly possible by those skilled in the art without departing fromthe scope of the invention, the detailed description is thus to beconsidered illustrative and not restrictive of the invention as claimedhereinbelow. For example, the discussion has generally used the term"ferroelectric" materials, however, the invention is generallyapplicable to any "high-dielectric constant oxide" and some suchmaterials are not ferroelectric and some not titanates. The term"high-dielectric constant oxides" as used herein is to mean oxides withdielectric constants of greater than 100, and preferably greater than1,000 (barium strontium titanate can have dielectric constants greaterthan 10,000). Many such oxides can be considered to be based on BaTiO₃and includes oxides of the general formula (Ba,Sr,Ca)(Ti,Zr,Hf)O₃. Manyother oxides of the general formula (K,Na,Li)(Ta,Nb)O₃ and still otheroxides such as (Pb,La)ZrTiO₃ or (Pb,Mg)NbO₃ or Bi₄ Ti₃ O₁₂ will alsowork. These oxides can also be doped with acceptors such as Al, Mg, Mn,or Na, or doners such as La, Nb, or P. Other non-germaniumsemiconductors can also be used in addition to silicon and galliumarsenide. As used herein, the term "semiconductor" is used to mean"non-germanium semiconductor".

What is claimed is:
 1. A method for fabricating a structure useful insemiconductor circuitry, comprising:growing a germanium layer directly(or indirectly) on a non-germanium semiconductor substrate or on anintervening layer which intervening layer is on a non-germaniumsubstrate; and depositing (a high-dielectric constant oxide) an oxidehaving a dielectric constant of greater than 100 on said germaniumlayer.
 2. The method of claim 1, wherein said germanium layer isepitaxially grown on said semiconductor substrate.
 3. The method ofclaim 1, wherein said substrate is silicon.
 4. The method of claim 3,wherein said high-dielectric constant oxide is epitaxially grown on saidgermanium layer.
 5. The method of claim 1, wherein said substrate isgallium arsenide.
 6. The method of claim 5, wherein said high-dielectricconstant oxide is epitaxially grown on said germanium layer.
 7. Themethod of claim 1, wherein a second germanium layer is grown on top ofsaid high-dielectric constant oxide.
 8. The method of claim 7, wherein aconducting layer is grown on said second germanium layer.
 9. The methodof claim 7, wherein an epitaxial conducting layer is grown on saidsecond germanium layer.
 10. The method of claim 1, wherein saidhigh-dielectric constant oxide is a titanate.
 11. The method of claim10, wherein said high-dielectric constant oxide is barium strontiumtitanate.
 12. The method of claim 10, wherein said high-dielectricconstant oxide is a lead or Bi-containing titanate and a buffer layer ofnon-lead-containing titanate is utilized between said germanium layerand said lead-containing titanate.
 13. The method of claim 1, whereinsaid high-dielectric constant oxide is a ferroelectric oxide.
 14. Themethod of claim 1, wherein said substrate is Si or GaAs.
 15. A methodfor fabricating a structure useful in semiconductor circuitry,comprising:growing a germanium layer directly (or indirectly) on anon-germanium semiconductor substrate or on an intervening layer whichintervening layer is on a non-germanium substrate; and depositing aferroelectric oxide on said germanium layer.
 16. The method of claim 1,wherein said dielectric constant is greater than
 1000. 17. The method ofclaim 1, wherein said oxide is selected from the group consisting of:(Ba,Sr,Ca)(Ti,Zr,Hf)O₃ ; (K,Na,Li)(Ta,Nb)O₃ ; (Pb,La)ZrTiO₃ ;(Pb,Mg)NbO₃ ; and Bi₄ Ti₃ O₁₂.
 18. The method of claim 16, wherein saidoxide is selected from the group consisting of: (Ba,Sr,Ca)(Ti,Zr,Hf)O₃ ;(K,Na,Li)(Ta,Nb)O₃ ; (Pb,La)ZrTiO₃ ; (Pb,Mg)NbO₃ ; and Bi₄ Ti₃ O₁₂. 19.The method of claim 1, wherein said oxide is doped.